Weather has always been the #1 topic of conversation for anyone in agriculture. Changing weather patterns have made it even more topical. Current research suggests that there could be many more days of high temperature, and that means a growing potential for heat stressed dairy cows. For those of us in the dairy business, this trend has the potential to dramatically impact all aspects of the industry from breeding to milk production.

Let’s Start with the Bad News First

Numerous studies have documented the challenges (Jordan, 2003; Rensis, 2003; West, 2004) which impact reproductive performance. Heat stressed cows eat less and this nutrition deficit results in prolonged postpartum anestrus and impaired embryonic development. In addition, this inadequate nutrient intake reduces body condition score and causes cessation of estrus cycles. Unfortunately, symptoms induced by heat stress gradually pile on and the ultimate result is that the success of gestation is severely compromised even after the weather has moderated.

Find Out Who Is NOT Cool

Monitoring respiration rate is a good tool for determining the level of heat stress. Aim for less than 75 breaths per minute in milking cows or 65 in dry. Rates above 65 breaths per minute in a dry cow indicate significant heat strain and require that action be taken for cooling.

Rectal temperature is another tool to detect heat stress. If the measurement is only taken once a day, it is best to take it in the afternoon (3:00 to 5:00 p.m.) when cows are most likely to be experiencing elevated body temperature. A more detailed method of monitoring for elevated body temperature can be done by placing computerized data logger in a blank CIDR device and then capturing the results every hour.

Heat Causes Ovary Dysfunction!

Beyond the obvious discomfort experienced by heat stressed cows, the entire reproduction program may be compromised if cows fail to come into heat or ovarian function fails.

More commonly, the frequency of mounting in hot weather is reduced, and the few mounts that do occur are missed. The high temperatures have a direct effect on feeding behavior, which can become so severely reduced that insufficient nutrients are available after milk production for the ovaries to start functioning during the first 6 weeks of lactation. Cows that calve immediately before or during hot weather are most likely to be affected. Check the records for each cow not observed in estrus. It is important to look at results of the veterinarian`s 30 to 75 day postpartum palpation record for each cow. Research shows that over 95% of all cows will have either a corpus luteum (CL or Yellow Body) or a follicle on one of their two ovaries at this stage of lactation. Any herd with fewer than 75% of cows cycling by 60 days after calving should have the energy level of the feed checked. Nutritionists working in Florida recommend increasing the energy concentration of the cow`s feed to account for reduced intake during hot weather.

Turn Down the Heat Stress. Watch the Temperature Humidity Index.

Providing shade over the feeding and watering area is another way to increase the feed intake of heat-stressed cows. Combinations of soakers, fans and shades that help cows maintain a body temperature around 101.5 degrees F over the course of the day are required when the THI (temperature humidity index) exceeds 68. Note that a THI of 68 is often not perceived as detrimental to cow performance, but studies show that it is the point where cows begin to have negative symptoms due to heat stress.

Researchers in Florida suggest that the body of a dairy cow begins to respond to warm temperatures in the environment at slightly over 70 degrees F. Recent studies show that the reproductive performance of cows does not suffer until environmental temperatures are over 90 degrees F. Additionally, the sun’s rays can directly have an adverse effect on fertility. Cows with black markings will absorb more heat from the sun’s rays, further elevating body temperature. Blood flow may be diverted from internal circulation to peripheral circulation in an attempt to reduce body temperature. The reduction in blood flow to internal organs including the uterus, oviducts and ovaries may reduce available nutrients and increase biochemical waste products at the tissue level. There are many good nutrition supplements specifically formulated to moderate the effects of heat stress.

Heat Stress and Heat Detection

Under the influence of heat stress, the duration and intensity of oestrus are reduced. There is a clear decrease in motor activity and other manifestations of oestrus such as mounting. Nobel et al. (1997) found that Holstein cows during the summer have 4.5 mounts per oestrus versus 8.6 per oestrus in winter. Higher incidence of silent heat and anoestrus is, therefore, one of the most often reported findings in cows exposed to high ambient temperatures. It is important to recognize the challenges of marginal heats and establish best practices to compensate. The use of timed breeding protocols could also help by ensuring that cows showing no signs of heat do not continuously get skipped when breeding. There are several, well researched timed breeding programs to pick from, so work with your veterinarian to implement one that works best for your dairy

Heat Affects Bulls Too

Heat stress can also cause infertility in bulls. Under normal conditions, testicles are kept a few degrees cooler than the body because heat interferes with proper sperm development, but during hot weather the body’s methods for keeping the testes cool are not adequate.

In some cases, producers decide to discontinue A.I. breeding because they are concerned that A. I. Performance declines in the summer. This is a big mistake because nature service bulls’ fertility suffers just as much or more than cow fertility so now heat stress has a detrimental effect on both sides of the reproductive process.

Heat Affects Semen

Heat stress causes hyperthermia of the scrotum and testes that can lead to poorer morphological and functional semen quality. Hansen (1997) reported deterioration of bull fertility caused by heat stress during the summer months. Semen quality declines markedly during extreme temperatures. There are carry-over effects, since the spermatogenic cycle is about 60 days from the time the cell is produced until it is out of the system. You might have problems with infertile bulls even up to two months after the hot weather ends.

Heat Causes Low Conception Rates

Assuming you do not have problems with the semen, heat stress problems may still result in conception rates falling below 20% during the summer months. Often, fertility will not return to normal until late October or November, even though environmental temperatures became unstressful in late September or early October.

The most common time for heat stress to cause embryonic loss is in the first week after mating (the embryo fails to attach to the uterus); though in some circumstances a pregnancy can be lost up to the first month of gestation. More rarely, heat stress can cause pregnancy loss in late gestation. Anything that stresses the cow or fetus at that time can trigger premature birth, which usually results in death of the calf. Some advisors advise against breeding during July, August or September reports Dr. Gary Williams with the Animal Reproduction Laboratory in Texas.

As things heat up, don’t let your profitability go down. Research shows that cooling cows during their dry period can keep the milk coming for months to come.

About The Author

Bullvine writer, Karen Hunt, is a dairy-farm wife who is an author, entrepreneur and champion of all things rural and family farm. Karen loves the wordy side of life. Husband, three children, eight grandchildren, thirty cows and Jack Russell terriers are her inspiration!